Numerous techniques for exploring the earth to acquire geophysical data are well known. Seismic surveys, however, are the best and most definitive geophysical means of structural representation currently in use. In some instances, seismic data can provide nearly direct indication of oil or gas. These indications, however, are very specialized and are intimately related to very local geological conditions. Reflection type seismic surveys are most common. In these reflection surveys seismic waves are induced in solid earth, reflected back to the surface by subsurface strata where the reflected waves are detected by a group of spaced apart receivers called geophones, and seismic signals representative of the detected waves are processed to form images of the subsurface.
Seismic exploration utilizes an acoustic source such as a dynamite explosion or a land vibrator to induce a seismic wave. The measured travel time of the seismic wave from the source to a receiver is used to produce a field record. Variations in travel time of acoustic waves produce time related field records that indicate the position of reflection surfaces, and correlation of a group of field records from appropriately spaced apart receivers produce a cross section that demonstrates the subsurface structure.
In practice, the detectability of a seismic signal representative of a reflected wave is limited by the energy of the received wave i.e. that is, by the product of its average power and its duration. Accordingly, many seismic systems utilize a relatively long pilot signal such as 3 to 15 seconds to drive a land vibrator to assure that high seismic wave energy is imparted into the earth, and then compress the long reflection signal to a short pulse to assure good resolution. This principle is known as pulse compression, and is achieved through cross-correlation of the received signal against the pilot signal.
Correlation techniques are well described e.g. in N. A. Anstey, "Correlation Techniques--A Review", Geophysical Prospecting, Vol. XII, No. 4, 1964. In simplified terms correlation techniques identify the presence of a seismic reflection in the time domain based on the sum obtained from the well known cross-correlation technique, i.e. solving the equation: ##EQU1## where g (t-.tau.) is an input sweep signal delayed by an amount (.tau.) and where r(t) is the measured reflection signal. In effect the pilot signal is "overlayed" onto the reflection signal at incremental delay times .tau.; at such delay times as the area under the curve representative of the above product is a maximum, the delay time equals the travel time of the reflected signal i.e. a two-way time is identified. Assuming that the wave velocities in the subsurface strata are known, the depth of the associated reflecting subsurface strata interface can be determined.
A land vibrator, 10 in FIG. 1, uses a pulse compressive method of seismic exploration known commercially as VIBROSEIS.RTM.. This system utilizes a reference or so called pilot sweep of sinusoidal signals of varying frequency, of selected band width and of selected time duration as illustrated in FIG. 2(A) to control injection of seismic acoustic waves into the earth. For example the sinusoidal frequency may vary from 10 to 48 hertz during a 4 second period in which the seismic energy is imparted at the surface of the earth.
In operating land vibrator exploration systems, energy is transmitted by vibrating a baseplate 12 in FIG. 1, which is large mass coupled to the solid earth and may be of the type mounted on a land vehicle, such that the vibrating plate 12 placed on the ground 14 is mechanically coupled to the earth by the weight of the entire vehicle. A hydraulic circuit 16 acts on reaction mass 18 causing the baseplate 12 to vibrate. At rest a mechanical system (not shown) serves to put the vehicle close to a limiting position where its supporting wheels leave the ground, at which point the ground 14 beneath the baseplate 12 is subjected to a constant force due to the overall weight of the generator and the vehicle carrying it. Since the vehicle should not be subjected to the vibrations generated by hydraulic circuit 16, the vehicle is decoupled from the baseplate 18 by shock absorbers such as conventional air bags, 20 and 22.
In operation, the pressure of fluid in the hydraulic circuit 16 is modulated as a function of time to comply with variations imposed by a "pilot" signal 24, which is shown in greater detail in the waveform of FIG. 2(a). The pilot signal is an alternating signal having a frequency which is continuously varying between limiting frequencies, and is applied to the hydraulic circuit 16 by any suitable means.
A recording sequence generally begins when emission of the wave starts, and it stops only after a period equal to the duration of the emission plus an idle period corresponding to the time taken for the waves to reach and travel back from the deepest reflector of interest between the emission and reception points. Various schemes have been employed to insure that the energy imparted into the earth is in phase with the long pilot signal used to drive the land vibrator so that the cross-correlation technique is effective for pulse compression. For example phase locking techniques are known where the pilot signal is phase locked to a signal generated by the land vibrator to maintain the seismic energy in phase with the pilot signal.
While these schemes have been generally successful for detecting energy contained in reflected seismic waves, a difficulty is encountered in cross-correlating a slowly varying sine wave that is injected into the earth and the reflected signal, in that harmonic energy may be present in both directions on the time shift axis of the correlation curve. These harmonics, which are in a large part non-repeating, represent a significant portion of the injected energy and in some cases mask small reflection signals. A land vibrator seismic system utilizing a reference signal for correlation against a received signal that does not require phase coherence with the pilot signal and that further could detect reflected harmonic energy would be highly desirable.
As used herein a correlation operator (CO) is a signal representative of the actual dynamics of the transmitted seismic energy injected into the earth by a land vibrator. This signal may be obtained, for example, from transducers which sense the motion of the vibrating baseplate of the land vibrator.
It is an object of this invention to utilize a reference signal for pulse compression of a received signal that does not require maintenance of phase relationship with the pilot signal.
It is another object of this invention to effectively utilize received harmonic energy.
It is a further object of this invention to increase the signal energy that is recovered in seismic systems that employ a long pilot signal.
It is a still further object of this invention to suppress interference caused by harmonic signal energy.
Additional objects, advantages and novel features of this invention will be apparent in the description which follows and in the appended claims and in the drawings.